Application Note No. 110/2013 Nitrogen & protein determination in corn, flour and soy KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376:

Transcription

1 Application Note No. 110/2013 Nitrogen & protein determination in corn, flour and soy KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376: Nitrogen and Protein Determination in Corn, Flour and Soy according to the Kjeldahl Method

2 1 Introduction An easy and reliable method for the determination of total nitrogen and protein in corn, flour and soy, according to ISO 20483:2006 and LFGB 64 L , is introduced below. The samples are digested using the KjelDigester K-449. The distillation and boric acid titration are performed with the KjelMaster K-375 with KjelSampler K-376. The combination of the new KjelDigester and the KjelMaster system K-375/K-376 increases the sample throughput. 2 Equipment KjelDigester K-449 (the parameters used are also valid for K-446) Scrubber K-415 TripleScrub ECO KjelMaster K-375 with KjelSampler K-376 Mixer, Retsch Grindomix GM200 Analytical balance (accuracy ± 0.1 mg) 3 Chemicals and Materials Chemicals: Sulfuric acid conc 98 %, Merck ( ) Titanium, BUCHI Kjeldahl Tablet ( ) Sodium hydroxide 32 %, Brenntag ( ) Boric acid 4 %, 400 g boric acid, Brenntag ( ) diluted to 10 L with deionized water, ph adjusted to 4.65 Sulfuric acid 0.1 mol/l, Fluka (35357) Neutralization solution for the Scrubber: 600 g sodium carbonate, calcined, technical, Synopharm ( ) about 2 ml ethanol and a spatula tip of bromthymol blue, Fluka (18460) diluted to 3 L with distilled water D/L tryptophan, assay 99 %, Fluka (162698) For a safe handling please pay attention to all corresponding MSDS! Samples: Corn with a protein content of 9 g/100 g [1] Wheat flour with a labelled protein content of 14 g/100 g Soy beans with a protein content of 35 g/100 g [1] The samples were purchased at a local supermarket. Application Note 110/2013 March /7

3 4 Procedure The determination of nitrogen and protein in corn, wheat flour and soy includes the following steps: Drying of the reference substance tryptophan [2, 3] Homogenization of the sample by grinding Digestion of the sample, using K-449 (K-446 respectively) Distillation and titration of the sample, using KjelMaster system K-375/K Digestion method tryptophan (verification of the method) 1. Start the KjelDigester K-449 according to the parameters listed in Table 2 2. Place 0.15 g tryptophan in a 300 ml sample tube 3. Add 2 Titanium Tablets and 15 ml of sulfuric acid (conc. 98 %) 4. Prepare additional blanks, chemicals without sample 5. Connect the Scrubber K-415 to the K-449 for absorbing the acid fumes created during digestion 6. Insert the rack with the samples into the cooling position and mount the suction module onto the samples, immediately start the digestion according to the parameters listed in Table Let the samples cool down when the digestion is completed. 4.2 Digestion method samples 1. Start the KjelDigester K-449 according to the parameters listed in Table 2 2. Place each sample in a 300 ml sample tube as described in Table 1 Table 1: Weight for each sample Sample Weight [g] Corn 1.0 Wheat flour 1.0 Soy beans Add 2 Titanium Tablets and 15 ml of sulfuric acid (conc. 98%) to each tube 4. Prepare additional blanks, chemicals without sample 5. Connect the Scrubber K-415 to the K-449 for absorbing acid fumes created during digestion 6. Insert the rack with the samples into the cooling position and mount the suction module onto the samples, immediately start the digestion according to the parameters listed in Table 2. Table 2: Temperature profile for digestion with the K-449 Step Temperature [ C] Time [min] Cooling 35 NOTE: If the liquid inside the sample tube is not clear and blue-green, digest for additional 15 min at 420 C. 7. Let the samples cool down when the digestion is completed. Application Note 110/2013 March /7

4 4.3 Distillation and titration Distill the samples according to the parameters listed in Table 3. Table 3: Distillation and titration with the KjelMaster system K-375/K-376 Method parameters KjelMaster K-375 H 2 O volume 50 ml Titration solution H 2 SO mol/l NaOH volume 60 ml Sensor type Potentiometric Reaction time 5 s Titration mode Online Distillation mode Fixed time Titration start time 120 s Distillation time 180 s Measuring mode Endpoint ph Stirrer speed distillation 5 Endpoint ph 4.65 Steam output 100 % Stirrer speed titration 7 Titration type Boric acid Titration start volume 0 ml Receiving solution vol. 50 ml Titration algorithm Optimal NOTE: The sample throughput for this application was increased by using the Titration mode Online. 4.4 Calculation The results are calculated as a percentage of nitrogen. In order to calculate the protein content of the sample, the nitrogen content is multiplied with a sample-specific protein factor. The following equations (1), (2), and (3) are used to calculate the results. For the reference substance, the purity of the tryptophan is considered in equation (4). w N (V Sample - V m Blank Sample ) z c f M 1000 N (1) %N = w N 100 % (2) %P = w N PF 100 % (3) %N 100 = P %NTry (4) w N V Sample V Blank : weight fraction of nitrogen : amount of titrant for the sample [ml] : mean amount of titrant for the blank [ml] z : molar valence factor (1 for HCl, 2 for H 2 SO 4 ) c : titrant concentration [mol/l] f : titrant factor (for commercial solutions normally 1.000) M N m Sample : molecular weight of nitrogen ( g/mol) : sample weight [g] 1000 : conversion factor [ml/l] %N : percentage of weight of nitrogen %N Try : percentage of weight of nitrogen corrected for the purity of reference substance tryptophan [%] Application Note 110/2013 March /7

5 %P : percentage of weight of protein P : purity of the reference substance tryptophan [%] PF : sample-specific protein factor (6.25 for corn, wheat flour and 5.71 for soy beans) 5 Results 5.1. Recovery of tryptophan The results of nitrogen determination and recovery for tryptophan analysis (assay > 99 %) are presented in Table 4. The nominal value of tryptophan is % nitrogen. The recoveries are within the specification of 99 %. [2, 3] Table 4: Results of the recovery of nitrogen in tryptophan Tryptophan msample [g] VSample [ml] %NTry Recovery [%] Sample Sample Sample Sample Average [%] Rsd [%] Protein determination in corn, wheat flour and soy beans The results of the determination of nitrogen and protein contents are presented in Tables 5 7. Table 5: Results of the determination of nitrogen and protein in corn (protein content 9 g/100 g [1]) Corn msample [g] VSample [ml] %N %P Sample Sample Sample Sample Average [%] Rsd [%] Table 6: Results of the determination of nitrogen and protein in wheat flour (labelled protein content 14 g/100 g) Wheat flour msample [g] VSample [ml] %N %P Sample Sample Sample Sample Average [%] Rsd [%] Application Note 110/2013 March /7

6 Table 7: Results of the determination of nitrogen and protein in soy beans (protein content 35 g/100 g [1]) Soy beans msample [g] VSample [ml] %N %P Sample Sample Sample Sample Average [%] Rsd [%] Comparison to Standard Methods This application note is based on the standard method ISO 20483:2006 and LFGB 64 L with minor differences. These differences are shown in Table 8. Table 8: Differences to the official regulations Application note Regulations Notes / Impact Sulfuric acid 15 ml 20 ml No impact, same ration of sulfuric acid/catalyst. Titration solution H 2 SO 4 0.2N H 2 SO 4 0.1N No impact consumption of the titration solution should be between 3-17 ml. 7 Conclusion The determination of nitrogen and protein in corn, wheat flour and soy beans using the KjelDigester K-449 and KjelMaster system K-375/K-376 provides reliable and reproducible results. These results correspond well to the labelled values with low relative standard deviations (rsd). The recovery with tryptophan was 99.7 % (rsd = 0.2 %), which was within the specification of 99 % [2, 3]. With the KjelDigester K-449 the digestion process (including preheating, digestion and cooling) is very fast and is fully automated. Together with the fully-automatic KjelMaster system K-375/K-376, the time to result is significantly reduced and it offers fully walk-away convenience. Application Note 110/2013 March /7

7 8 References [1] Souci Fachmann Kraut: Die Zusammensetzung der Lebensmittel. Nährwert-Tabellen, CRS Press, 7., revidierte und ergänzte Auflage [2] ISO 20483:2006 Cereals and pulses Determination of the nitrogen content and calculation of the crude protein content Kjeldahl method [3] LFGB 64 L Kjeldahl Calculator App Operation Manual of KjelDigester K-446/K-449 Operation Manual of Scrubber K-415 Operation Manual of KjelMaster system K-375/K376 Application Note 110/2013 March /7